US 7883927 B2
Methods and systems for sorting nanostructures, such as nanodot or nanotubes, are described. The sorting of the nanostructures removes remnants of the nanotube fabrication from the mixture or bundle of material. The sorting includes suspending the mixture in a plasma, which separated the nanostructures and remnant material. A motive force, such as gas flow or laser, is applied to the suspended nanostructures and remnants such that the larger material moves out of the plasma while the smaller material remains trapped in the plasma.
1. A method, comprising:
providing a bundle of nanotubes;
suspending the nanotubes in a plasma to provide plasma-suspended nanotubes; and
sorting the plasma-suspended nanotubes according to size and separating the plasma-suspended nanotubes from remnant material using a motive force.
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10. A method, comprising:
fabricating a bundle of nanotubes using an electric arc technique;
suspending the nanotubes in a plasma to provided plasma-suspended nanotubes; and
sorting the plasma-suspended nanotubes according to size and separating the plasma-suspended nanotubes from remnant material resulting from the fabricating.
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22. An integrated circuit fabrication method, comprising:
providing a bundle of nanotubes;
suspending the nanotubes in a plasma to provide plasma-suspended nanotubes;
sorting the plasma-suspended nanotubes according to size and separating the plasma-suspended nanotubes from remnant material to keep a usable batch of nanotubes; and
applying the usable batch of nanotubes to an integrated circuit.
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This invention pertains to nanotube fabrication, and more particularly to fabrication systems and methods.
Nanotubes are cylindrical structures that have a diameter of about five to about 300 nanometers and exhibit unique properties. The principal type of nanotube is the carbon nanotube. Since carbon nanotubes were discovered in NEC Laboratories in 1991, the pace of research into the intriguing properties of carbon nanotubes has accelerated. Some of the proposed intriguing applications of carbon nanotubes include field emitters of flat panel displays, memory devices, transistors, mechanical reinforcing elements, and biomedical applications. While nanotubes have promising properties, there is a significant impediment to their use in commercial applications. Namely, the current techniques for fabricating nanotubes result in batches of nanotubes that do not have uniform properties and include other undesirable materials, such as carbonaceous materials. Other techniques take too long for their use in industry. Accordingly, the nanotubes must be sorted to have uniform properties for use in an application. That is, if the application is for an integrated circuit, the nanotubes must be sorted according to their electrical properties and physical dimensions, so that the nanotubes fit the design parameters. However current techniques for nanotube sorting do not provide the yields based on either time or quantity. Accordingly, there is a need for improved nanotube and nanostructure sorting
The present invention includes methods and systems for nanostructure, such as nanotube, sorting. Other nanostructures include, nut are not limited to nanocrystals and nanoparticles. One method includes providing a bundle of nanotubes, suspending the nanotubes in a plasma, and sorting the nanotubes. The bundle of nanotubes, in an embodiment, are a plurality of single walled carbon nanotubes. Sorting the nanotubes includes applying a gas flow to the suspended nanotubes. Sorting the nanotubes includes lazing the suspended nanotube, in an embodiment. Fabricating the bundle of nanostructures includes using an electric arc technique, in an embodiment. The sorted nanotubes can be used to fabricate components of an integrated circuit.
The systems include a chamber to receive a bundle of nanotubes, which, in an embodiment, includes other remnant material. A plasma source is positioned to create a plasma in the chamber to suspend the bundle of raw nanotubes. A motive force to selectively move one group of nanotubes, having a certain size, is provided. The motive force moves a group of larger nanotubes from the smaller nanotubes. The smaller nanotubes will be preferentially left behind stuck in the plasma sheath. In an embodiment, the motive source is a gas flow. In an embodiment, the motive force includes a laser system.
These and other aspects, embodiments, advantages, and features will become apparent from the following description and the referenced drawings.
In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. The terms wafer and substrate used in the following description include any base semiconductor structure. Both are to be understood as including silicon-on-sapphire (SOS) technology, silicon-on-insulator (SOI) technology, thin film transistor (TFT) technology, doped and undoped semiconductors, epitaxial layers of a silicon supported by a base semiconductor structure, as well as other semiconductor structures well known to one skilled in the art. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.
Nanotube fabrication 105 as shown in
Reactor 200 includes means for supplying energy to the reactable constituents or compounds in the process gases in chamber 201 on the surface of the substrate 202. The supplied energy causes the reactable constituents to react or decompose and deposit a thin seed film 202B onto the upper surface of substrate layer 202A. In one embodiment, the supplied energy includes thermal energy supplied by heat lamps 206. Heat lamps 206 heat the substrate and/or gases and/or chamber 201 according to the teachings of the present invention. In the illustrated example, lamps 206 are positioned in the base of chamber 201. Heat lamps 206 emit a significant amount of near-infra red radiation that passes through susceptor 207 to heat substrate 202. Alternatively, susceptor 207 is heated by heat lamps 206. The substrate 202 is heated by conduction from susceptor 207. The heat lamps 206 may be placed at alternate locations according to the parameters of the specific nanotube fabrication process being performed according to the present invention. Supplying heat to the chamber 201 is important as forming nanotubes, and, in particular, single walled carbon nanotubes is a high temperature dependent process. The heat sources as described herein are adapted to raise the temperature of the chamber up to 700 deg. C. High temperatures typically assist in creating well ordered nanotubes. It is within the scope to grow nanotubes and nanostructures at temperatures of about 450 deg. C.
Another embodiment supplies reaction energy by a radio frequency (RF) generator 208 as shown in
Chamber 201 includes a scraper 220 that is adapted to scrape the nanostructure layer 202C and other remnant materials from the surface of substrate 202. The scraper 220 may further scrape the seed layer 202B from the substrate 202. The scraper 220 delivers a mass of nanotubes, reaction remnants on seed layer for further processing. In order to use the nanotubes in subsequent fabrication, the nanotubes must be separated from the remnants and sorted by size.
In general, the energy sources 206 and 208 are intended to provide sufficient reaction energy in a region near the surface of substrate 202 to cause decomposition and/or reaction of the constituents of the present gas to deposit a seed layer 202B, if not already present. Moreover, energy sources 206 and 208 provide energy to grow nanotube or nanodots on seed layer 202B. One of ordinary skill in the art will understand upon reading the disclosure that any one, combination, or equivalent of the above can be employed to provide the necessary reaction energy.
Reactor 200 includes a material source 230 that provides deposition material. An energy source 240 is positioned to impart energy to the material source 230. This energy removes material from the source 230. The material is deposited on the substrate 202. The material source 230 and energy source 240 may be a physical deposition system, such as a sputtering system.
Furthermore, reactor 200 includes associated control apparatus (not shown) for detecting, measuring and controlling process conditions within reactor 200. Associated control apparatus include, as examples, temperature sensors, pressure transducers, flow meters, control valves, and control systems. Control systems include computational units such as programmable logic controls, computers and processors. Associated control apparatus further include other devices suitable for the detection, measurement and control of the various process conditions described herein.
Nanotube sorter 110 as shown in
In a further embodiment, the nanostructures, such as nanotubes, are suspended using the techniques and structures described herein. The nanoparticles are essentially even distributed an then released, i.e., dropped, onto a workpiece. The workpiece is a substrate in an embodiment. The workpiece further includes other composite materials. This evenly distributes the nanoparticles on the substrate. The even distribution will improve the properties of the resulting structure, for example, structural strength.
The bulk material formed by the fabrication of nanotubes, including nanotubes, particulate material, etc. is sorted, steps 620, 630. In an embodiment, the bulk or raw fabrication material that include the nanotubes is not emmersed in a solution. Instead, the bulk material is suspended in a plasma, step 620. A motive force is applied to suspended material to separate the bulk material into is components, e.g., particulate and nanotubes, step 630. This technique relies on the fact that when two particles are charged the repulsive force of the negative charges separate the particles. The motive force contacts the suspended, separated materials in the plasma. The drag force exerted on these particles would sort the particles based on their cross-section. The motive force is a gas flow, in an embodiment. The motive force is a force exerted by lasers in an embodiment. While described in conjunction with nanotubes, it will be recognized that the present method may be used to evenly disperse metal nanodots onto a substrate.
The sorting steps 620, 630 are believed to be based on the following principles. The capacitance of an isolated cylinder (nanotube) in plasma is governed by its geometry and the debye length of the plasma. This capacitance is derived as follows, with reference to
Electrostatic repulsive forces are typically connected only to the surface charge supported by the two nanostructures and could depend on the particle material. Plasma environment and local surface topography determines the equilibrium charge supported by nanostructures. For the sake of simplicity, the overall charge will be taken to be twice as high as the charge supported by a single particle in the same plasma (although the charge might be a little less than this value due to shadowing effect, but would not differ by an order of magnitude). The plasma-surface potential difference within the sheath is, usually, a repulsive one with an absolute value Vs (from a few hundreds of Vs, for a cathodic surface to a few tens of Vs for electrically isolated surfaces). The electric field and particle charge are functions of the particle position. A rough, overestimated, value of both parameters is obtained by using the mean value of electric field E in the sheath and the equilibrium charge Qp attached to the particle in the plasma bulk conditions. This means that the particle is lying in the sheath field but near the sheath limit. E will be expressed as Vs/Ls, where Vs is the sheath voltage and Ls is the sheath depth given by the Child-Langmuir approximation. The corresponding expression is
The charge distribution and polarization effects in this system depend on the nature of the particle material. As the electric field in the particle volume is reduced by partial cancellation of the anti-parallel electric fields associated with the two symmetric charge distributions, polarization effects are also reduced in this system. Assuming that the resulting repulsing force is given by an approximation between two cases of a perfect dielectric system (no charge mobility on the surface) and a metallic one (charge located at the maximum possible distance). The force between two infinitely thin (r<<I) long rods with a charge “q” uniformly distributed on its surface (not a dipole) is calculated as follows:
The minimum condition for the two tubes to remain stuck together is: Fr≦Fvw (Vander Waal force) [1.12]. If this condition [1.12] is violated then the two nanostructures, e.g., carbon nanotubes separate. The amount of charge held by the particle depends on the plasma parameters such as pressure, composition and ionization source. Such parameters can be controlled externally. Hence if we suspend the nanostructures (CNT) in plasma or simply drop CNTs into a plasma and form CNT dusty plasma the tubes would naturally separate when the condition [1.12] is violated. In an embodiment, where the nanostructures are carbon nanotubes, a helium based plasma would be avoided, in an embodiment, because helium tends to be chemically activate with carbon in the case of the nanostructures being carbon nanotubes or carbon nanostructures. (it is used in the production of CNTs) In an embodiment, a helium atmosphere or plasma is present when carbon nanotubes are formed.
In step 630, a motive force is provided to move and sort the separated nanostructures suspended in the plasma. It is known that for the thinner particles (≈20 nm) the gas flow effect will be reduced because the gas drag force varies as the square of the particle size, i.e., radius≈rp 2. Hence, if the gas flow within the nanostructure containing dusty plasma chamber is maintained constant then the larger particles are pushed towards outlet of chamber, typically connected to the pumping line while the smaller dust particles remain trapped within the plasma sheath. In this way smaller nanostructures are sorted from larger nanostructures. Once the separation of the larger nanostructures from the smaller nanostructures is complete, the remaining smaller nanostructures self assemble in a regular pattern within the plasma sheath that may be manipulated for other applications. If this process is repeated further sorting of nanostructures is possible.
Another way to push the nanostructures 1000 in the dusty plasma is by using laser beams 1001 from laser source 1002 as shown in
Nanotubes, in particular carbon nanotubes are fabricated in environments where remnants of materials used to fabricate the nanotubes remain. The nanotubes and remnants tend to stick together. This is one of the drawbacks to the use of nanotubes in commercial applications. Another drawback is the nonuniform length of nanotubes. Accordingly, there is a desire to sort and clean the nanotubes. The techniques described herein achieve these goals, as well as others that would be apparent to those of skill in the art. Generally, the present technique relies on the principle that like charged particles will repel each other and separate. A motive force is added to move the separated nanotubes and remnants. This motion will sort the nanotubes. The present technique is generally described for use with nanotubes but is also applicable to other nanostructures, such as nano-dots, nano-crystals, and nano-balls, etc. be used with the teachings herein.
Embodiments of the invention may be implemented in one or a combination of hardware, firmware, and software. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by at least one processor to perform the operations described herein. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium may include read-only memory (ROM), random-access memory (RAM), magnetic disc storage media, optical storage media, flash-memory devices, electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others.
The Abstract is provided to comply with 37 C.F.R. Section 1.72(b) requiring an abstract that will allow the reader to ascertain the nature and gist of the technical disclosure. It is submitted with the understanding that it will not be used to limit or interpret the scope or meaning of the claims.
All publications, patents, and patent documents are incorporated by reference herein, as though individually incorporated by reference.
In the foregoing detailed description, various features are occasionally grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments of the subject matter require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the detailed description, with each claim standing on its own as a separate preferred embodiment.
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